The present invention relates in general to inkjet printing systems and more specifically to system and method for automatically calibrating the print contrast of an inkjet printing system by using an optical-electronic sensor to detect an ink drop volume.
Inkjet printing systems are popular and common in the computer field. These printing systems are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Inkjet printings systems produce high-quality print, are compact and portable, and print quickly and quietly because only ink strikes a print medium (such as paper).
An inkjet printing system produces a printed image by printing a pattern of individual dots (or pixels) at specific defined locations of an array. These dot locations, which are conveniently visualized as being small dots in a rectilinear array, are defined by the pattern being printed. The printing operation, therefore, can be pictured as the filling of a pattern of dot locations with dots of ink.
Inkjet printing systems print dots by ejecting a small volume of ink onto the print medium. These small ink drops are positioned on the print medium by a moving carriage assembly that supports a printhead assembly (or printhead cartridge) containing ink-drop generators. The carriage assembly traverses over the print medium surface and positions the printhead assembly depending on the pattern being printed. An ink supply, such as an ink reservoir, supplies ink to the drop generators. The drop generators are controlled by a microprocessor or other controller and eject ink drops at appropriate times upon command by the microprocessor. The timing of ink drop ejections typically corresponds to the pixel pattern of the image being printed.
In general, the drop generators eject ink drops through a nozzle or an orifice by rapidly heating a small volume of ink located within a vaporization or firing chamber. The vaporization of the ink drops typically is accomplished using an electric heater, such as a small thin-film (or firing) resistor. Ejection of an ink drop is achieved by passing an electric current through a selected firing resistor to superheat a thin layer of ink located within a selected firing chamber. This superheating causes an explosive vaporization of the thin layer of ink and an ink drop ejection through an associated nozzle of the printhead.
Color inkjet printing systems usually have either a separate printhead for each color of ink or have a single printhead assembly fluidically coupled to ink reservoirs each containing a different ink color. Ideally the volume of each ink drop ejected from the nozzle of either printhead configuration is uniform and has minimal variation between drops. In reality, however, the ink drop volumes of inkjet printing systems can vary significantly due in part to manufacturing variances. For printing systems having a separate printhead for each ink color, ink drop volume variance can cause a shift in both hue and print contrast. For printing systems having a single printhead coupled to multiple ink colors, ink drop volume variance can cause a shift in print contrast. Print contrast shift is undesirable because the image printed by the printing system will not accurately reproduce the true colors of the original image. In other words, a shift in print contrast reduces the accuracy and reliability of the printing system. Thus, there exists a need for a printing system capable of alleviating any variance in ink drop volume and the resulting shift in print contrast.
To overcome the limitations in the prior art as described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a print contrast calibration system for an inkjet printing system that provides a automatic calibration of print contrast. This automatic calibration system determines ink drop volume using an optical-electronic sensor and adjusts the print contrast to compensate for any variance in ink drop volume. As use in this application, print contrast calibration includes all types of contrast calibration including hue calibration.
The print contrast calibration system of the present invention includes: (1) an optical-electronic sensor that obtains test pattern data from a test pattern printed on a print media; (2) a standard test module that creates standard test curves for a particular printing system, preferably during the product development stage; (3) a standard calibration curve generation module that creates standard calibration curves, preferably during the product development stage; (4) an installation module that uses standard test data and test pattern data to determine a drop volume score for a printhead assembly installed in the printing system; and (5) a printing module that creates a calibration curve for a particular printhead assembly based on its drop volume score and the standard calibration curves. The installation module is run automatically each time a new printhead assembly is installed in the printing system or whenever the user desires to start the calibration process. Once the drop volume score is obtained the printing module uses the score each time the printing system prints. The data for the standard test module and the standard calibration curve generation module preferably are generated during the product development stage, stored in a memory device and accessible by the remainder of the print calibration system. The print contrast calibration system of the present invention is able to automatically adjust the print contrast in a printing system without any user intervention to alleviate any variance in print contrast between printed images.
The present invention is also embodied in a method for calibrating print contrast in a printing system. The method includes storing a set of standard test curves, comparing the standard test curves with a printhead assembly test curve to determine a drop volume score and setting the print contrast by creating a calibration curve based on the drop volume score and the standard calibration curves.
Other aspects and advantages of the present invention as well as a more complete understanding thereof will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Moreover, it is intended that the scope of the invention be limited by the claims and not by the preceding summary or the following detailed description.